After reading this chapter, the student will be able to:
The vertebral column acts as a modified elastic rod, providing rigid support and flexibility (48). The spine is a complex structure that provides a connection between the upper and lower extremities (64). There are 33 vertebrae in the vertebral column, 24 of which are movable and contribute to trunk movements. The vertebrae are arranged into four curves that facilitate support of the column by offering a springlike response to loading (37). These curves provide balance and strengthen the spine.
Seven cervical vertebrae form a convex curve to the anterior side of the body. This curve develops as an infant begins to lift his or her head; it supports the head and assumes its curvature in response to head position. The 12 thoracic vertebrae form a curve that is convex to the posterior side of the body. The curvature in the thoracic spine is present at birth. Five lumbar vertebrae form a curve convex to the anterior side, which develops in response to weight bearing and is influenced by pelvic and lower extremity positioning. The last curve is the sacrococcygeal curve, formed by five fused sacral vertebrae and the four or five fused vertebrae of the coccyx. Figure 7-1 presents the curvature of the whole spine as seen from the side and the rear.
The junction where one curve ends and the next one begins is usually a site of great mobility, which is also vulnerable to injury. These junctions are the cervicothoracic, thoracolumbar, and lumbosacral regions. Additionally, if the curves of the spine are exaggerated, the column will be more mobile, and if the curves are flat, the spine will be more rigid. The cervical and lumbar regions of the spinal column are the most mobile, and the thoracic and pelvic regions are more rigid (37).
Besides offering support and flexibility to the trunk, the vertebral column has the main responsibility of protecting the spinal cord. As illustrated in Figure 7-2, the spinal cord runs down through the vertebrae in a canal formed by the body, pedicles, and pillars of the vertebrae, the disc, and a ligament (the ligamentum flavum). Peripheral nerves exit through the intervertebral foramen on the lateral side of the vertebrae, forming aggregates of nerve fibers and resulting in segmental innervations throughout the body.
FIGURE 7-1 The vertebral column is both strong and flexible as a result of the four alternating curves. We are born with the thoracic and sacrococcygeal curves. The cervical and lumbar curves form in response to weight bearing and muscular stresses imposed on them during infancy.
FIGURE 7-2 The vertebral column protects the spinal cord, which runs down the posterior aspect of the column through the vertebral foramen or canal. Spinal nerves exit at each vertebral level.
The trunk, as the largest segment of the body, plays an integral role in both upper and lower extremity function because its position can significantly alter the function of the extremities. Trunk movement or position can be examined as a whole, or it can be examined by observing the movements or position of the different regions of the vertebral column or movement at the individual vertebral level. This chapter examines both the movement of the trunk as a whole and the movements and function within each region of the spine. The structural characteristics of the vertebral column are presented first, followed by an examination of the differences between the three regions of the spine: the cervical, thoracic, and lumbar.
The Vertebral Column
The functional unit of the vertebral column, the motion segment, is similar in structure throughout the spinal column, except for the first two cervical vertebrae, which have unique structure. The motion segment consists of two adjacent vertebrae and a disc that separates them (Fig. 7-3). The segment can be further broken down into anterior and posterior portions, each playing a different role in vertebral function.
Motion Segment: Anterior Portion
The anterior portion of the motion segment contains the two bodies of the vertebrae, the intervertebral disc, and the anterior and posterior longitudinal ligaments. The two
bodies and the disc separating them form a cartilaginous joint that is not found at any other site in the body.
FIGURE 7-3 The vertebral motion segment can be divided into anterior and posterior portions. The anterior portion contains the vertebral bodies, intervertebral disc, and ligaments. The posterior portion contains the vertebral foramen, neural arches, intervertebral joints, transverse and spinous processes, and ligaments.
Each vertebral body is tube shaped and thicker on the front side (15), where it absorbs large amounts of compressive forces. It consists of cancellous tissue surrounded by a hard cortical layer and has a raised rim that facilitates attachment of the disc, muscles, and ligaments. Also, the surface of the body is covered with hyaline cartilage, forming articular end plates into which the disc attaches.
Separating the two adjacent bodies is the intervertebral disc, a structure binding the vertebrae together while permitting movement between adjacent vertebrae. The disc is capable of withstanding compressive forces as well as torsional and bending forces applied to the column. The roles of the disc are to bear and distribute loads in the vertebral column and to restrain excessive motion in the vertebral segment. The load transmitted via the intervertebral discs distributes stress uniformly over the vertebral end plates and is also responsible for most of the mobility in the spine (32). Lateral, superior, and cross-sectional views of the disc are presented in Figure 7-4.
Each disc consists of the nucleus pulposus and the annulus fibrosus. The nucleus pulposus is a gel-like, spherical mass in the central portion of the cervical and thoracic discs and toward the posterior in the lumbar discs. The nucleus pulposus is 80% to 90% water and 15% to 20% collagen (12), creating a fluid mass that is always under pressure and exerting a preload to the disc. The nucleus pulposus is well suited for withstanding compressive forces applied to the motion segment.
During the day, the water content of the disc is reduced with compressive forces applied during daily activities, resulting in a shortening of the column by about 15 to 25 mm (1). The height and volume of the discs are reduced by about 20%, causing the disc to bulge radially outward and increase the axial loading on the posterior joints (1). At night, the nucleus pulposus imbibes water, restoring height to the disc. In elderly individuals, the total water content of the disc is lower (approximately 70%), and the ability to imbibe water is reduced, leaving a shorter vertebral column.
FIGURE 7-4 The intervertebral disc bears and distributes loads on the vertebral column. The disc consists of a gel-like central portion, the nucleus pulposus, which is surrounded by rings of fibrous tissue, the annulus fibrosus.
The nucleus pulposus is surrounded by rings of fibrous tissue and fibrocartilage, the annulus fibrosus. The fibers of the annulus fibrosus run parallel in concentric layers but are oriented diagonally at 45° to 65° to the vertebral bodies (39,95). Each alternate layer of fibers runs perpendicular to the previous layer, creating a crisscross pattern similar to that seen in a radial tire (35). When rotation is applied to the disc, half of the fibers tighten, and the fibers running in the other direction will be loose.
The fibers that make up the annulus fibrosus consist of 50% to 60% collagen, providing the tensile strength in the disc (12). As a result of aging and maturation, the collagen is remodeled in the disc in response to changes in loading. This results in thicker annular fibers with higher concentrations of collagen fibers in the anterior disc area and thinner annular fibers in the lateral posterior portion of the disc because the fibers are less abundant. Fibers from the annulus fibrosus attach to the end plates of the adjacent vertebral bodies in the center of the segment and attach to the actual osseous material at the periphery of the disc (85). The fiber directions in the annulus fibrosus limit rotational and shearing motion between the vertebrae. The pressure on the tissue of the peripheral layer maintains the interspace between the end plates of the adjoining vertebrae (32). Tension is maintained in the annulus fibrosus by the end plates and by pressure exerted outward from the nucleus pulposus. The pressure tightens the outer layer and prevents radial bulging of the disc. Loss of disc tissue, such as occurs in aging, may impair spine function because of an increase in radial bulging, compression of the joints, or a reduction in space for the nerve tissue in the foramen (32).
The disc is both avascular and aneural, except for some sensory input from the outer layers of the annulus fibrosus. Because of this, healing of a damaged disc is unpredictable and not very promising.
The intervertebral disc functions hydrostatically when it is healthy, responding with flexibility under low loads and stiffly when subjected to high loads. When the disc is loaded in compression, the nucleus pulposus uniformly distributes pressure through the disc and acts as a cushion. The disc flattens and widens and the nucleus pulposus bulges laterally as the disc loses fluid. This places tension on the annulus fibers and converts vertical compression force to tensile stress in the annulus fibers. The tensile stress absorbed by the annulus fibers is 4 to 5 times the applied axial load (60).
There are two weak points where disc injury is likely when subjected to high loads. First, the cartilage end plates, to which the disc is attached, are supported only by a thin layer of bone and thus are subject to fracture. Second, the posterior annulus is thinner and not attached as firmly as other portions of the disc, making it more vulnerable to injury (95).
The pressure in the disc increases linearly with increased compressive loads, with the pressure 30% to 50% greater than the applied load per unit area (15). The greatest change in disc pressure occurs with compression. During compression, the disc loses fluid, and the fiber angle increases (39). The disc is very resilient to the effects of a compressive force and rarely fails under compression. The cancellous bone of the vertebral body yields and fractures before the disc is damaged (39).
Movements such as flexion, extension, and lateral flexion generate a bending force that causes both compression and tension. With this asymmetrical loading, the vertebral body translates toward the loaded side, where compression develops, and the fibers are stretched on the other side, resulting in tension force.
In flexion, the vertebrae tilt anteriorly, forcing the nucleus pulposus posteriorly, creating a compression load on the anterior portion of the disc and a tension load on the posterior annulus. In extension the opposite occurs, as the upper vertebrae tilt posteriorly, driving the nucleus pulposus anteriorly and placing tensile pressure on the anterior fibers of the annulus.
FIGURE 7-5 When the trunk flexes, extends, or laterally flexes, compressive force develops to the side of the bend and tension force develops on the opposite side.
In lateral flexion, the upper vertebrae tilt to the side of flexion, generating compression on that side and tension on the opposite side. Figure 7-5 illustrates disc behavior in flexion, extension, and lateral flexion.
As the trunk rotates, both tension and shear develop in the annulus fibrosus of the disc (Fig. 7-6). The half of the annulus fibers that are oriented in the direction of the rotation become taut, and the rest, which are oriented in
the opposite direction, slacken. This increases the intradis-cal pressure, narrows the joint space, and creates a shear force in the horizontal plane of rotation and tension in fibers oriented in the direction of the rotation. The peripheral fibers of the annulus fibrosus are subjected to the greatest stress during rotation (85).
FIGURE 7-6 When the trunk rotates, half of the fibers of the annulus fibrosus become taut, and the rest relax. This creates tension force in the fibers running in the direction of the rotation and shear force across the plane of rotation.
The final structures of the anterior portion of the vertebral segment are the longitudinal ligaments running along the spine from the base of the occiput to the sacrum. The ligaments that act on the vertebral column are illustrated in Figure 7-7. The anterior longitudinal ligament is a very dense, powerful ligament that attaches to both the anterior disc and the vertebral bodies of the motion segment. This ligament limits hyper extension of the spine and restrains forward movement of one vertebra over another. It also maintains a constant load on the vertebral column and supports the anterior portion of the disc in lifting (35).
The posterior longitudinal ligament runs down the posterior surface of the vertebral bodies inside the spinal canal and connects to the rim of the vertebral bodies and the center of the disc. The posterolateral aspect of the segment is not covered by this ligament, adding to the vulnerability of this site for disc protrusion. It is broad in the cervical region and narrow in the lumbar region. This ligament offers resistance in flexion of the spine.
Motion Segment: Posterior Portion
The posterior portion of the vertebral motion segment includes the neural arches, intervertebral joints, transverse and spinous processes, and ligaments (Figs. 7-7 and 7-8). The neural arch is formed by the two pedicles and two laminae, and together with the posterior side of the vertebral body, they form the vertebral foramen, in which the spinal cord is located. The bone in the pedicles and laminae is very hard, providing good resistance to the large tensile forces that must be accommodated. Notches above and below each pedicle form the intervertebral foramen, through which the spinal nerves leave the canal.
Projecting sideways at the union of the laminae and the pedicles are the transverse processes, and projecting posteriorly from the junction of the two laminae is the spinous process. The spinous and transverse processes serve as attachment sites for the spinal muscles running the length of the column.
The two synovial joints, termed the apophyseal joints, are formed by articulating facets on the upper and lower border of each lamina. The superior articulating facet is concave and fits into the convex inferior facet of the adjacent vertebra, forming a joint on each side of the vertebrae. The articulating facets are oriented at different angles in the cervical, thoracic, and lumbar regions of the
spine, accounting for most of the functional differences between regions. These differences are discussed more specifically in a later section of this chapter.
FIGURE 7-7 Ligaments of the spine. A. There are a number of longitudinal ligaments that run the length of the spine. B. The thoracic region of the spine has specialized ligaments that connect the ribs to the vertebrae. C. In the cervical region, specialized ligaments connect the vertebrae to the occipital bone.
The apophyseal joints are enclosed within a joint capsule and have all of the other characteristics of a typical synovial joint. Depending on the orientation of the facet joints, these joints can prevent the forward displacement of one vertebra over another and also participate in load bearing. In the hyper extended position, these joints bear 30% of the load (48). They also bear a significant portion of the load when the spine is flexed and rotated (30). Highest pressures in the facet joints occur with combined torsion, flexion, and compression of the vertebrae (10). The apophyseal joints protect the discs from excessive shear and rotation (1).
Five ligaments support the posterior portion of the vertebral segment (Fig. 7-7). The ligamentum flavum connects adjacent vertebral arches longitudinally, attaching laminae to laminae. This ligament has elastic qualities, allowing it to deform and return to its original length. It elongates with flexion of the trunk and contracts in extension. In the neutral position, it is under constant tension, imposing continual tension on the disc.
The supraspinous and the interspinous ligaments both run from spinous process to spinous process and resist both shear and forward bending of the spine. Finally, the intertransverse ligaments, connecting transverse process to transverse process, resist lateral bending of the trunk. The role of all of the intervertebral ligaments is to prevent excessive bending (1).
FIGURE 7-8 The posterior portion of the spinal motion segment is responsible for a significant amount of spinal support and restriction owing to its ligaments and structure. The posterior portion contains the only synovial joint in the spine, the apophyseal joint, which joins the superior and inferior facets of each vertebra.
FIGURE 7-9 The cervical vertebrae (A) have two unique vertebrae, the atlas (top right) and the axis (B), that are very different from a typical vertebra (C) and have specialized functions of supporting the head. (Reprinted with permission from
Sobotta . R. Putz, R. Pabst [Eds.]. Atlas of Human Anatomy, Vol. 2, Trunk, Viscera, Lower Limb.Philadelphia: Lippincott Williams & Wilkins, Figs. 720, 723, 725, 727.
Structural and Movement Characteristics of Each Spinal Region
The cervical region has two vertebrae, the atlas (C1) and the axis (C2), that have structures unlike those of any other vertebra (Fig. 7-9). The atlas has no vertebral body and is shaped like a ring with an anterior and a posterior arch. The atlas has large transverse processes with transverse foramen through which blood supply travels. The atlas has no spinous process. Superiorly, it has a fovea, or dishlike depression, that holds the occiput of the skull.
The articulation of the atlas with the skull is called the atlantooccipital joint. At this joint, the head nods on the
spine because this joint allows free sagittal plane movements. This joint allows approximately 10° to 15° of flexion and extension (97) and no lateral flexion or rotation (87).
The weight of the head is transferred to the cervical spine via C2, the axis. The axis has a modified body with no articulating process on the superior aspect of the body and pedicles. Instead, the articulation with the atlas occurs via a pillar projecting from the superior surface of the axis that fits into the atlas and locks the atlas into a swivel or pivoting joint. The pillar is referred to as the odontoid process or dens.
FIGURE 7-10 The vertebrae in each region (cervical, thoracic, and lumbar) have common structural features with unique regional variations, as seen here in the anterior (A), posterior (B), and lateral (C) views. (Reprinted with permission from
Sobotta . R Putz, R. Pabst [Eds.]. Atlas of Human Anatomy, Vol. 2, Trunk, Viscera, Lower Limb.Philadelphia: Lippincott Williams & Wilkins, Figs. 708-710.
The articulation between the atlas and the axis is known as the atlantoaxial joint and is the most mobile of the cervical joints, allowing approximately 10° of flexion and extension, 47° to 50° of rotation, and no lateral flexion (97). This joint allows us to turn our head and look from one side to the other. In fact, this articulation accounts for 50% of the rotation in the cervical vertebrae (97).
The remainder of the cervical vertebrae support the weight of the head, respond to muscle forces, and provide mobility. C3-C7 vertebrae have structures in the anterior and posterior compartments similar to those of the typical vertebrae. The bodies of the cervical vertebrae are small and about half as wide side to side as they are front to back. The cervical vertebrae also have short pedicles, bulky articulating processes, and short spinous processes. The transverse processes of the cervical vertebrae have a foramen where the arteries pass through. This is not found in other regions of the vertebral column. Figure 7-10 illustrates size, shape, and orientation differences across the regions of the spinal cord. A closer examination of structural differences between the cervical, thoracic, and lumbar vertebrae is presented in Figure 7-11.
FIGURE 7-11 The cervical, thoracic, and lumbar vertebrae differ from each other. From the cervical to the lumbar region the bodies of the vertebrae become larger, and the transverse processes, spinous processes, and apophyseal joints all change their orientation.
The articulating facets in the cervical vertebrae face 45° to the transverse plane and lie parallel to the frontal plane (82), with the superior articulating process facing posterior and up and the inferior articulating processes facing anterior and down. In contrast to other regions of the vertebral column, the intervertebral discs are
smaller laterally than the bodies of the vertebrae. The cervical discs are thicker ventrally than dorsally, producing a wedge shape and contributing to the lordotic curvature in the cervical region.
Because of the short spinous processes, the shape of the discs, and the backward and downward orientation of the articulating facets, movement in the cervical region is greater than in any other region of the vertebral column. The cervical vertebrae can rotate through approximately 90°, flex 20° to 45° to each side, flex through 80° to 90°, and extend through 70° (87). Maximum rotation in the cervical vertebrae occurs at C1-C2, maximum lateral flexion at C2-C4, and maximum flexion and extension at C1-C3 and C7-T1. Also, all cervical vertebrae move simultaneously in flexion.
In addition to the ligaments that support the whole vertebral column, some specialized ligaments are found in the cervical region. The locations and actions of these ligaments are presented in Figure 7-7.
One of the most restricted regions of the vertebral column is the thoracic vertebrae. Moving down the spinal column, the individual vertebrae increase in size; thus, the twelfth thoracic vertebra is larger than the first one. The bodies become taller, and the thoracic vertebrae have longer pedicles than the cervical vertebrae (Fig. 7-11). The transverse processes on the thoracic vertebrae are long, and they angle backward, with the tips of the transverse processes posterior to the articulating facets. On the back of the thoracic vertebrae are long spinous processes that overlap the vertebrae and are directed downward rather than posteriorly, as in other regions of the spine.
The connection of the thoracic vertebrae to the ribs is illustrated in Figure 7-12. The thoracic vertebrae articulate with the ribs via articulating facets on the body of each vertebrae. Full facets are located on the bodies of T1 and T10-T12, and demifacets are located on T2-T9 to connect with the ribs. The thoracic vertebrae are supported by the ligaments presented earlier, along with four others that support the attachment between the ribs and the vertebral body and transverse processes (Fig. 7-7).
FIGURE 7-12 The thoracic region is restricted in movement because of its connection to the ribs, which connect to a demifacet on the body of the thoracic vertebrae and a facet on the transverse process.
The apophyseal joints between adjacent thoracic vertebrae are angled at 60° to the transverse plane and 20° to the frontal plane, with the superior facets facing posterior and a little up and laterally and the inferior facets facing anteriorly, down, and medially (Fig. 7-11). Compared with the cervical vertebrae, the thoracic intervertebral joints are oriented more in the vertical plane.
The movements in the thoracic region are limited primarily by the connection with the ribs, the orientation of the facets, and the long spinous processes that overlap in the back. Range of motion in the thoracic region for flexion and extension combined is 3° to 12°, with very limited motion in the upper thoracic (2° to 4°) that increases in the lower thoracic to 20° at the thoracolumbar junction (10,97).
Lateral flexion is also limited in the thoracic vertebrae, ranging from 2° to 9° and again increasing as one progresses down through the thoracic vertebrae. Whereas in the upper thoracic vertebrae, lateral flexion is limited to 2° to 4°, in the lower thoracic vertebrae, it may be as high as 9° (10,97).
Rotation in the thoracic vertebrae ranges from 2° to 9°. Rotation range of motion is opposite to that of flexion and lateral flexion because it is maximum at the upper levels (9°) and is reduced at the lower levels (2°) (10,97).
The intervertebral discs in the thoracic region have a greater ratio of disc diameter to height of the disc than any other region of the spine. This reduces the tensile stress imposed on the vertebrae in compression by reducing the stress on the outside of the disc (60). Thus, disc injuries in the thoracic region are not as common as in other regions of the spinal column.
The large lumbar vertebra is the most highly loaded structure in the skeletal system. Figure 7-11 illustrates the characteristics of the lumbar vertebrae. The lumbar vertebrae are large, with wider bodies side to side than front to back. They also are wider vertically in the front than in the back. The pedicles of the lumbar vertebrae are short; the spinous processes are broad; and the small transverse processes project posteriorly, upward, and laterally. The discs in the lumbar region are thick; as in the cervical region, they are thicker ventrally than dorsally, contributing to an increase in the anterior concavity in the region. Frobin et al. (32) reported that the ventral disc height of the lumbar vertebrae remains fairly constant in the age range of 16 to 57 years, but there are gender differences and different disc heights at different levels of the vertebrae. The lumbar vertebrae are typically higher in males. Also, the highest disc height is found at L4-L5 and L5-S1.
The apophyseal joints in the lumbar region lie in the sagittal plane; the articulating facets are at right angles to
the transverse plane and 45° to the frontal plane (97). The superior facets face medially, and the inferior facets face laterally. This changes at the lumbosacral junction, where the apophyseal joint moves into the frontal plane and the inferior facet on L5 faces front. This change in orientation keeps the vertebral column from sliding forward on the sacrum.
The lumbar region is supported by the ligaments that run the full length of the spine and by one other, the iliolumbar ligament (Fig. 7-7). Another important support structure in the region is the thoracolumbar fascia, which runs from the sacrum and iliac crest up to the thoracic cage. This fascia offers resistance and support in full flexion of the trunk. The elastic tension in this fascia also assists with initiating trunk extension (35).
The range of motion in the lumbar region is large in flexion and extension, ranging from 8° to 20° at the various levels of the vertebrae (10,97). Lateral flexion at the various levels of the lumbar vertebrae is limited, ranging from 3° to 6°, and there is also very little rotation (1° to 2°) at each levels of the lumbar vertebrae (10,97). However, the collective range of motion in the lumbar region ranges from 52° to 59° for flexion, 15° to 37° for extension, 14° to 26° for lateral flexion and 9° to 18° of rotation (93). A review of the range of motion at each level of the vertebral column is presented in Figure 7-13.
The lumbosacral joint is the most mobile of the lumbar joints, accounting for a large proportion of the flexion and extension in the region. Of the flexion and extension in the lumbar vertebrae, 75% may occur at this joint, with 20% of the remaining flexion at L4-L5 and 5% at the other lumbar levels (77).
FIGURE 7-13 Range of motion at the individual motion segments of the spine is shown. The cervical vertebrae can produce the most range of motion at the individual motion segments. (Redrawn from
White, A. A., Panjabi, M. M. . Clinical Biomechanics of the Spine. Philadelphia: Lippincott.
Movement Characteristics of the Total Spine
Motion in the spinal column is very small between each vertebra, but as a whole, the spine is capable of considerable range of motion. Motion is restricted by the discs and the arrangement of the facets, but motion can occur in three planes via active muscular initiation and control (90).
The movement characteristics of the total spine are presented in Figure 7-14. For the total spinal column, flexion and extension occur through approximately 110° to 140°, with free movement in the cervical and lumbar regions and limited flexion and extension in the thoracic region. The axis of rotation for flexion and extension lies in the disc unless there is considerable disc degeneration, which can move the axis of rotation out of the disc. Flexion of the whole trunk occurs primarily in the lumbar vertebrae through the first 50° to 60° and is then moved into more flexion by forward tilt of the pelvis (31). Extension occurs through a reverse movement in which first the pelvis tilts posteriorly and then the lumbar spine extends.
When flexion begins, the top vertebra slides forward on the bottom vertebra and the vertebra tilts, placing compressive force on the anterior portion of the disc. Both ligaments and the annulus fibers absorb the compressive forces.
FIGURE 7-14 The range of motion at the individual motion segment level is small, but in combination, the trunk is capable of moving through a significant motion range. Flexion and extension occur through approximately 110° to 140°, primarily in the cervical and lumbar region, with a very limited contribution from the thoracic region. The trunk rotates through 90°, with movement occurring freely in the cervical region, and with accompanying lateral flexion in the thoracic and lumbar regions. The trunk laterally flexes through 75° to 85°.
On the back side, the superior portions of the apophyseal joints slide up on the lower facets, creating compression force between the facets and shear force across the face of the facets. These forces are controlled by the posterior ligaments, the capsules surrounding the apophyseal joints, posterior muscles, fascia, and posterior annulus fibers (85). The full flexion position is maintained and supported by the apophyseal capsular ligaments, intervertebral discs, supraspinous and interspinous ligaments, lig-amentum flavum, and passive resistance from the back muscles, in that order (3).
Lateral flexion range of motion is about 75° to 85°, mainly in the cervical and lumbar regions (Fig. 7-14). During lateral flexion, there is a slight movement of the vertebrae sideways, with disc compression to the side of the bend. Lateral flexion is often accompanied by rotation. In a relaxed stance, the accompanying rotation is to the opposite side of lateral flexion, that is, left rotation accompanying right lateral flexion.
If the vertebra is in full flexion, the accompanying rotation occurs to the same side, that is, right rotation accompanying right lateral flexion. This can vary by region of the spine. Also, an inflexible person usually performs some lateral flexion to obtain flexion in the trunk (2).
Rotation occurs through 90°, is free in the cervical region, and occurs in the thoracic and lumbar regions in combination with lateral flexion (Fig. 7-14). Generally, rotation is limited in the lumbar region. Right rotation in the thoracic or lumbar region is accompanied by some left lateral flexion.
The apophyseal joints are in a close-packed position in spinal extension, except for the top two cervical vertebrae, which are in a close-packed position in flexion. The total spine is in a close-packed position and is rigid during the military salute posture with the head up, shoulders back, and the trunk vertically aligned (35).
The flexibility of the regions of the trunk varies and is determined by the intervertebral discs and the angle of articulation of the facet joints. As pointed out earlier, mobility is highest at the junction of the regions. Mobility also increases in a region in response to restriction or rigidity elsewhere in the vertebral column.
FIGURE 7-15 A. In the normal standing posture, there is slight curvature in the lumbar region. B. The first 50° of flexion takes place in the lumbar vertebrae as they flatten. C.The continuation of flexion is a result of an anterior tilt of the pelvis.
Combined Movements of the Pelvis and Trunk
The relationship of the movements of the pelvis to the movements of the trunk is discussed in Chapter 6. The movement synchronization between the pelvis and the trunk is referred to as the lumbopelvic rhythm. As shown in Figure 7-15, the lumbar curve reverses itself, flattens out (flexes), and curves in the opposite direction as trunk flexion progresses. This continues to a point at which the low back is rounded in full flexion of the trunk. Accompanying the movements in the lumbar vertebrae are flexion of the sacrum, anterior tilt of the pelvis, and extension of the sacrum. The pelvis also moves backward as weight is shifted over the hips.
Lumbar activity is maximum through the first 50° to 60° of flexion, after which anterior pelvic rotation becomes the predominant factor that increases trunk flexion. On the return extension movement, pelvic posterior tilt dominates the initial stages of the extension, and lumbar activity reverses itself, dominating the later stages of trunk extension. The pelvis also moves forward as weight is shifted.
Movement relationships between the pelvis and the trunk during trunk rotation or lateral flexion are not as clear cut as in flexion and extension because of restrictions to the movement introduced by the lower extremity. The pelvis moves with the trunk in rotation and rotates right with trunk right rotation unless the lower extremity is forcing a rotation of the pelvis in the opposite direction. In this case, the pelvis may remain in the neutral position or rotate to the side exerting greater force.
Similarly, in lateral flexion of the trunk, the pelvis lowers to the side of the lateral flexion unless resistance is offered by the lower extremity, in which case the pelvis rotates to the opposite side (Fig. 7-16). The accompanying pelvic movements are determined by the trunk movement and the unilateral or bilateral positioning of the lower extremity.
The movement relationship between the pelvis and the trunk becomes complex when lower extremity movement, such as running, is performed in which the individual has different sequences of one limb moving on the ground and a limb moving off the ground. The lumbar spine flexes slightly and the pelvis tilts posteriorly during the loading
phase with a quick reversal to lumbar extension and anterior pelvic tilt by midstance. Peak lumbar extension and anterior pelvic tilt occur right after toe-off. In the frontal plane, the spine laterally flexes to the right side, and the pelvis tilts to the left side during right foot contact and loading. This is followed by lumbar spine lateral flexion to the left side as the pelvis begins to elevate and tilt to the right until toe-off. Finally, the lumbar spine and the pelvis both rotate to the right with a right limb contact. The lumbar spine and pelvis rotate to the left during the support phase, but not at the same time (78).
FIGURE 7-16 In walking and running, the trunk laterally flexes to the support side, but the pelvis lowers to the nonsupport side because of resistance offered by the lower extremity.
Trunk extension is an important movement used to raise the trunk and to maintain an upright posture. The muscles typically get stronger as you come down the spine. The muscles actively used to extend the trunk also play dominant roles in trunk flexion; thus, it seems logical to first review the extensors.
The spinal extensors are graphically presented, and insertion, action, and nerve supply information is provided in Figure 7-17.
Numerous small muscles constitute the extensor muscle group. They can be classified into two groups, the erector spinae (iliocostalis, longissimus, spinalis) and the deep posterior, or paravertebral, muscles (intertransver-sarii, interspinals, rotatores, multifidus). These muscles run up and down the spinal column in pairs and create extension if activated as a pair or rotation or lateral flexion if activated unilaterally. Also, a superficial layer of muscle includes the trapezius and the latissimus dorsi. Although both the trapezius and the latissimus dorsi can influence trunk motion, they are not discussed in this chapter.
The three erector spinae muscles constitute the largest mass of muscles contributing to trunk extension. Extension is also produced by contributions from the deep vertebral muscles and other muscles specific to the region. These deep muscles contribute to trunk extension and other trunk movements, and they support the vertebral column, maintain rigidity in the column, and produce some of the finer movements in the motion segment (85).
There are some other muscles besides the erector spinae and the deep posterior muscle groups specific to each region. Figure 7-17 provides a full description of these muscles.
The erector spinae muscles are thickest in the cervical and lumbar regions, where most of the extension in the spine occurs. The multifidus is also thickest in the cervical and lumbar regions, adding to the muscle mass for generation of a trunk extension force.
The erector spinae and the multifidus muscles are 57% to 62% type I muscle fibers but also have types IIa and IIb fibers, making them functionally versatile so they can generate rapid, forceful movements while still resisting fatigue for the maintenance of postures over long periods (89). In addition to providing the muscle force for extension of the trunk, these muscles provide posterior stability to the vertebral column, counteract gravity in the maintenance of an upright posture, and are important in the control of forward flexion (71).
Flexion of the trunk is free in the cervical region, limited in the thoracic region, and free again in the lumbar region. Unlike the posterior extensor muscles, the anterior flexors do not run the length of the column. Flexion of the lumbar spine is created by the abdominals with assistance from the psoas major and minor. The flexion force of the
abdominals also creates what little flexion there is in the thoracic vertebrae. The abdominals consist of four muscles: the rectus abdominus, internal oblique, external oblique, and transverse abdominus (see Fig. 7-17).
FIGURE 7-17 Muscles acting on the spine, including the surface anatomy (A) and anterior muscles (B) of the trunk; deep anterior neck muscles (C); surface anatomy (D) and muscles (E) of the lateral neck region; deep posterior muscles of the neck and upper back (F), and surface anatomy (G) with corresponding superficial (H), deep (I), and pelvic(J) muscles of the posterior trunk.
The internal and external oblique muscles and the transverse abdominus attach into the thoracolumbar fascia covering the posterior region of the trunk. When they contract, they place tension on the fascia, supporting the low back and reducing the strain on the posterior erector spinae muscles (9,71). The obliques are active in erect posture and in sitting, possibly stabilizing the base of the spine (83). The activity of the obliques drops off in a stooped standing posture as the load is transferred to other structures (83).
The transverse abdominus wraps around the trunk similar to a support belt and supports the trunk while assisting with breathing. The transverse abdominus applies tension to the linea alba, which is a fibrous connective tissue that runs vertically down the front that separates the rectus abdominus into right and left halves. If the linea alba is stabilized by transverse abdominus contraction, the obliques on the opposite side can act on the trunk. This
muscle is also important for pressurizing the abdominal cavity (83) in activities such as coughing, laughing, defecation, and childbirth.
The abdominals consist of 55% to 58% type I fibers, 15% to 23% type IIa fibers, and 21% to 22% type IIb fibers (89). This fiber makeup, similar to that in the erector spinae muscles, allows for the same type of versatility in the production of short, rapid movements and prolonged movements of the trunk.
Two other muscles contribute to flexion in the lumbar region. First is the powerful flexor acting at the hip, the iliopsoas muscle, which attaches to the anterior bodies of the lumbar vertebrae and the inside of the ilium. The iliopsoas can initiate trunk flexion and pull the pelvis forward, creating lordotic posture in the lumbar vertebrae. Additionally, if this muscle is tight, an exaggerated anterior tilt of the pelvis may develop. If the tilt is not counteracted by the abdominals, lordosis increases, compressive stress on the facet joints develops, and the intervertebral disc is pushed posteriorly.
The second muscle found in the lumbar region is the quadratus lumborum, which forms the lateral wall of the abdomen and runs from the iliac crest to the last rib (Fig. 7-17). Although positioned to be more of a lateral flexor, the quadratus lumborum contributes to the flexion movement. It is also responsible for maintaining pelvic position on the swing side in gait (35).
When a person is standing or sitting upright, there is intermittent activity in both the erector spinae muscles and the internal and external obliques. The iliopsoas, on the other hand, is continuously active in the upright posture, but the rectus abdominus is inactive (81).
Flexion in the thoracic region, which is limited, is developed by the muscles of the lumbar and cervical regions. In the cervical region are five pairs of muscles that produce flexion if both muscles in the pair are contracting. If only one of the muscles in the pair contracts, the result is motion in all three directions, including flexion, rotation, and lateral flexion (85). The insertions, actions, and nerve supplies of these muscles are presented in Figure 7-17.
Standing Toe Touch
Movement into the fully flexed position from a standing posture is initiated by the abdominals and the iliopsoas muscles. After the movement begins, it is continued by the force of gravity acting on the trunk and controlled by the eccentric action of the erector spinae muscles. There is a gradual increase in the level of activity in the erector spinae muscles up to 50° to 60° of flexion as the trunk flexes at the lumbar vertebrae (6).
As the lumbar vertebrae discontinue their contribution to trunk flexion, the movement continues as a result of the contribution of anterior pelvic tilt. The posterior hip muscles, hamstrings, and gluteus maximus eccentrically work to control this forward tilt of the pelvis. As the trunk moves deeper into flexion, the activity in the erector spinae diminishes to total inactivity in the fully flexed position. In this position, the posterior ligaments and the passive resistance of the elongated erector spinae muscles control and resist the trunk flexion (48). The load on the ligaments in this fully flexed position is close to their failure strength (31), placing additional importance on loads sustained by the thoracolumbar fascia and the lumbar apophyseal joints.
As the trunk rises back to the standing position through extension, the movement is initiated by a contraction of the posterior hip muscles, gluteus maximus, and hamstrings, which flex and rotate the pelvis posteriorly. The erector spinae are active initially but are most active through the last 45° to 50° of the extension movement (71).
The erector spinae muscles are more active in the raising than in the lowering phase, being very active in the initial parts of the movements and again at the end of the extension movement, with some diminished activity in the middle of the movement. The abdominals can also be active in the return movement as they serve to control the extension movement (48).
Trunk Lateral Flexion
Lateral flexion of the spine is created by contraction of muscles on both sides of the vertebral column, with most activity on the side to which the lateral flexion occurs. The most activity in lateral flexion of the trunk occurs in the lumbar erector spinae muscles and the deep intertransver-sarii and interspinales muscles on the contralateral side. The multifidus muscle is inactive during lateral flexion. If load is held in the arm during lateral flexion, there is also an increase in the thoracic erector spinae muscles on the opposite side.
The quadratus lumborum and the abdominals also contribute to lateral flexion. The quadratus lumborum on the side of the bend is in a position to make a significant contribution to lateral flexion. The abdominals also contract as the lateral flexion is initiated and remain active to modify the lateral flexion movement.
In the cervical spine, lateral flexion is further facilitated by unilateral contractions of the sternocleidomastoid, scalenes, and deep anterior muscles. Lateral flexion is quite free in the cervical region.
The rotation of the trunk is more complicated in terms of muscle actions because it is produced by muscle actions on both sides of the vertebral column. In the lumbar region, the multifidus muscles on the side to which the rotation occurs are active, as are the longissimus and ilio-costalis on the other side (8). The abdominals exhibit a similar pattern because the internal oblique on the side of the rotation is active, and the external oblique on the opposite side of the rotation is also active.
Strength of the Trunk Muscles
The greatest strength output in the trunk can be developed in extension, averaging values of 210 Nm (newton-meters) for males (56). Reported trunk flexion strength is 150 Nm, or approximately 70% of the strength of the extensors. Lateral flexion is 145 Nm, or 69% of the extensor strength, and rotation strength is 90 Nm, or 43% of the extensor values (56). Female strength values are approximately 60% of the values recorded for males. In fact, other studies have shown women to be capable of generating only 50% of the lifting force of men for lifts low to the ground and 33% of the male lifting force for lifts high off the ground (99). In the cervical region, women have demonstrated as much as 20% to 70% less strength than men (19).
Taking into consideration all things such as forces generated by intraabdominal pressure, ligaments, and other structures, the total extensor moment is slightly greater than the flexor moment (72). The abdominals contribute to one third of the flexor moments, and the erector spinae contribute half of the extensor moments. In rotation, the abdominals dominate, with some contribution by the small posterior muscles (72).
Trunk position plays a significant role in the development of strength output in the various movements. Trunk flexion strength, measured isometrically, has been shown to improve by approximately 9% when measured from a position of 20° of hyperextension (85). Isometric trunk extension strength, measured from a position of 20° of trunk flexion, is 22% greater compared with a 20° trunk flexion position (85). Higher trunk flexion and extension strength values can also be achieved if the measurement is made with the person seated rather than supine or prone.
The strength of the trunk is significantly altered in a dynamic situation. There is a reported 15% to 70% increase in trunk moments during dynamic exertions accompanied by increases in antagonist and agonist muscle activity, an increase in intraabdominal pressure, an increase in spinal load, and a reduction in the capacity of the muscles to respond to external loads (24). Because of higher levels of coactivity, there is greater loading on the spinal structures without contributing to the ability to offset external moments. It is suggested that trunk velocity and acceleration, especially in multiple directions, may be more accurate discriminators of low-back disorders than just range of motion because of the diminished strength and functional capacity that accompany the coactivation in faster dynamic movements (24).
Strength output while lifting an object using the trunk extensors also diminishes when there is greater horizontal distance between the feet and the hands placed on the object (33). In fact, the forces applied vertically to an object held away from the body are about half those of a lift completed with the object close to the body. Additionally, an increase in the width of a box decreases lifting capacity, and an increase in the length of a box has been shown to have no influence (33).
Lifting an object by pulling up at an angle reduces the load at the elbow, shoulder, and lumbar and hip regions but increases it at the knees and ankles. This type of lift decreases the compressive force on the lumbar vertebrae 9% to 15% (34). Also, 16% more weight can be lifted by a more freestyle lift than the traditional straight-back, bent-knee lift (34).
Posture and Spinal Stabilization
Efficiency of motion and stresses imposed on the spine are very much determined by the posture maintained in the trunk as well as trunk stability. Positioning of the vertebral segments is so important that a special section on posture and spinal stabilization is warranted.
The spine is stabilized by three systems, including a passive musculoskeletal system, an active musculoskeletal subsystem, and the neural feedback system (67). The passive subsystem includes the vertebrae, facet articulations, joint capsules, intervertebral disks, and spinal ligaments. The active system includes the muscles and tendons that stabilize the spine, and the neural subsystem provides control. Stability in the spine increases and decreases with the demands placed on the structure. Stability decreases during periods of decreased muscle activity and increases when joint compressive forces increase (20). The smaller deep muscles of the spine control posture and the relationship of each vertebra to each other (73), and the larger, superficial muscles move the spine and disperse loads from the thorax to the pelvis. For stability, the spine requires activity in the small postural muscles to be stable.
Muscles that play an important role in spinal stabilization include the transverse abdominus, multifidus, erector spinae, and internal oblique. The transverse abdominus circles the trunk like a belt and increases intraabdominal pressure and spinal stiffening. It is one of the first muscles to be active in both unexpected and self-loaded conditions (22). The multifidus is organized to act at the level of each vertebra and is active continuously in upright positions (92) and can make subtle adjustments to the vertebrae in any posture (51). The erector spinae is better suited for control of spinal orientation by nature of its ability to produce extension (73). Finally, the internal oblique works with the transverse abdominus to increase intraabdominal pressure.
To maintain an upright posture in standing, the S-shaped spine acts as an elastic rod in supporting the weight. A continuous forward bending action is imposed on the trunk
in standing because the center of gravity lies in front of the spine. As a result of the forward bending action on the trunk, the posterior muscles and ligaments must control and maintain the standing posture.
There is more erector spinae activity in an erect posture than in a slouched posture. In the slouched posture, most of the responsibility for maintaining the posture is passed onto the ligaments and capsules. Any disruption in the standing posture or any postural swaying is controlled and brought back into alignment by the erector spinae, abdominals, and psoas muscles (66). All of these muscles are slightly active in standing, with more activity in the thoracic region than the other two regions (8).
Posture in the sitting position requires less energy expenditure and imposes less load on the lower extremity than standing. Prolonged sitting, however, can have deleterious effects on the lumbar spine (85). Unsupported sitting is similar to standing, such that there is high muscle activity in the thoracic regions of the trunk with accompanying low levels of activity in the abdominals and the psoas muscles (7).
The unsupported sitting position places more load on the lumbar spine than standing because it creates a backward tilt, a flattening of the low back, and a corresponding forward shift in the center of gravity (48). This places load on the discs and the posterior structures of the vertebral segment. Sitting for a long time in the flexed position may increase the resting length of the erector spinae muscles (71) and overstretch the posterior ligamentous structures. A slouched sitting posture generates the largest disc pressures. Higher seating height can decrease the compressive force on the disks because of a more vertical posture, but increased loads are put on the lower extremity.
Biomechanical factors related to static work postures; seated and standing work postures; frequent bending and twisting; and lifting, pulling and pushing are some of the risk factors for back injuries. Because of the high incidence of back related injuries in the workplace, it is important to understand both causes and preventive measures.
Working postures can greatly influence the accumulation of strain on the low back (54), and both standing and sitting postures have appropriate uses in the workplace (28). A standing posture is preferred when the worker cannot put his or her legs under the work area and when more strength is required in the work task such as lifting or applying maximal grip forces. Strain can be reduced in a standing posture by using floor mats, using a foot rest, making sure the work station has adequate foot clearance, and wearing proper shoes (28). Muscle fatigue can also be reduced with several short breaks over the course of the work day. One of the most important factors for both standing and sitting is to avoid prolonged static postures.
Continuous flexion positions are a cause of both lumbar and cervical flexion injuries in the workplace. These postures can be eliminated by raising the height of the work station so that no more than 20° of flexion is present (85). The use of a footrest can also relieve strain.
Lifting tasks in the workplace can be the source of the low back pain, so guidelines should be established to reduce the risk. For example, the weights of objects being lifted should be lowered as lift frequency, lift distance, and object size increases. Weights should be lowered when lifting above shoulder height. Proper lifting technique, which involves maintaining a neutral spine, keeping the load close to the pelvis, avoiding trunk flexion and extension, and lifting with the lower extremity with a controlled velocity, is optimal for most tasks. In cases in which an object is awkwardly placed or when an object is in motion before the lift, it may be necessary to use a jerking motion. This places less torque on the lumbar extensor muscles (54).
Similar to the standing and working postures, the risk of injury from lifting can be minimized with regular breaks and by varying the work tasks (54). A fully flexed spine should be avoided in any lift because of the changes imposed on the major lumbar extensors. The fully flexed spine reduces the moment arms for the extensor muscles, decreases the tolerance to compressive loads, and transfers load from the muscles to the passive tissues (53). The workplace also has a high incidence of twisting, in which the spine undergoes combined flexion and lateral bending. This posture maximally stretches the posterolateral structures, particularly the annulus (39). Twisting in the upright posture is limited by contact at the facet joints, but twisting in a flexed posture disengages the facet joints and shifts the resistance to the annulus fibrosus (50,70).
In seated work environments, a well-designed chair is important for providing optimal support because unsupported sitting results in disc pressures that are 40% higher than a standing posture (28). Prolonged sitting in a slouched flexion position maximally loads the iliolumbar ligaments because the loss of lumbar lordosis and the positioning of the upper body weight behind the ischial tuberosities (84). In supported sitting, the load on the lumbar vertebrae is lessened. A chair back reclined slightly backward and including lumbar support creates a seated posture that produces the least load on the lumbar region of the spine.
A lumbar backrest with free shoulder space is recommended for reducing some of the load. Higher backrests may not be effective and because the ribcage is stiff. Backrests above the level just below the scapulae are not necessary (83). The work setting should be evaluated to determine high-risk lifting tasks such as repetitive bending, twisting, pushing, pulling, or lift and carry tasks.
Postural deviations in the trunk are common in the general population (Fig. 7-18). In the cervical region, the curve is concave to the anterior side. This curve should be
small and lie over the shoulder girdle. The head should be above the shoulder girdle. When the cervical curve is accentuated to the anterior side, lordosis is said to be present. Thus, cervical lordosis is an increase in the curve in the cervical region, often concomitant with exaggerated curves in other regions of the spine.
FIGURE 7-18 A. The ideal posture is one in which the curves are balanced but not exaggerated. B. Curves can become exaggerated. C. Lateral deviation of the spine, scoliosis, can create serious postural malalignment throughout the whole body.
In the thoracic region, the curvature is concave to the posterior side. A rounded-shoulder posture may cause thoracic kyphosis, a common postural disorder in this region. The kyphotic thoracic region is also associated with osteoporosis and several other disorders.
The lumbar region, curving anteriorly, is subjected to forces that may be created by an exaggerated lumbar curve, termed lumbar lordosis or hyperlordosis. This accentuated swayback position is often created by anterior positioning of the pelvis or by weak abdominals. In the lumbar region, it is also not uncommon to have a flat back with decreased lumbar curve. This has been associated with a pelvis that is inclined upward at the front or with muscle tightness and rigidity in the spine.
The most serious of the postural disorders affecting the spine is scoliosis, a lateral deviation of the spine. The curve can be C shaped or S shaped depending on the direction and the beginning and ending segments. C-shaped scoliosis is designated when the deviation occurs in one region only. For example, a convex curve to the left in the cervical region is a left cervical C-shaped curve. In an S-shaped curve, the lateral deviation occurs in different regions and in opposite directions, as with a right thoracic, left lumbar convexity. Rotation can accompany the lateral deviation, creating a very complex postural malalignment. The cause of scoliosis is unknown, and it is more prevalent in females than in males.
The muscles around the trunk are active during most activities as they stabilize the trunk, move the trunk into an advantageous position for supplementing force production, or assist limb movement. Because the low back is a common site of injury in sports and in the workplace, special attention should be given to exercises that strengthen and stretch this part of the trunk. Endurance training for the back muscles may be one of the better avenues for preventing back injury (54). Trunk exercises should also be evaluated for negative impact on trunk function and structure. A sample of trunk exercises in illustrated in Figure 7-19.
Trunk exercises should take place with the spine in the neutral position and use co-contraction of the abdominals. Co-contraction of the abdominals and the erector spinae increases spinal stiffness and stability, allowing for a better response to spinal loading (94). Spinal compression is increased with co-contraction, however, so the levels of co-contraction may need to be lessened for individuals with back pain who would be negatively affected by more compression.
Exercises creating excessive lordosis or hyperextension of the lumbar vertebrae should be avoided because they put excessive pressure on the posterior element of the spinal segment and can disrupt the facets or the posterior arch. Examples of such exercises are double-leg raises, double-leg raises with scissoring, thigh extension from the prone position, donkey kicks, back bends, and ballet arches. When selecting an exercise for the trunk, one should pay attention to its risks. The supine position produces the least amount of load on the lumbar vertebrae. The load in the supine position increases substantially, however, if the abdominals and the iliopsoas are activated.
The trunk flexors are usually exercised with some form of trunk or thigh flexion exercise from the supine position so that these muscles can work against gravity. Trunk flexion exercises should be evaluated for their effectiveness and safety. Three variations of trunk flexor exercises are shown in Figure 7-20. The abdominals are more active in trunk raising activities than in double leg-raising activities (5). However, abdominal activity is important in leg raising exercises to stabilize the pelvis and maintain rigidity in the trunk. Altering leg position between a bent or straight leg does not appear to significantly change the total muscle activity levels in the abdominals (5). The bent knee positions do engage the hip flexors to a higher degree because of the reduced moment arm and decreased force capacity of the iliopsoas.
There is no one single best exercise for all abdominals at once. The best exercise for the rectus abdominus that maximizes activation and minimizes psoas activation is the curl, and the best exercise for the obliques is a side-supported position held either isometrically or dynamically (44).
FIGURE 7-19 Sample stretching and strengthening exercises for selected muscle groups.
FIGURE 7-20 The abdominals and hip flexors are used in different ways in exercises depending on the position of the trunk and hip joint.
Extension of the trunk is usually developed through some type of lift using the legs and back. Figure 7-19 provides some examples of extensor exercises for stretching and strengthening these muscles.
Two basic types of lifts, the leg lift and the back lift activate the erector spinae. The leg lift is the squat or dead-lift exercise in which the back is maintained in an erect or slightly flexed posture and the knees are flexed. This lift has the least amount of erector spinae activity and imposes the lowest shear and compressive forces on the spine (74). The leg lift is begun with posterior tilt of the pelvis initiated by the gluteus maximus and the hamstrings. The erector spinae can be delayed and not involved until later in the leg lift, when the extension is increased. The delay is related to the magnitude of the weight being lifted, and the muscles usually do not become active until the initial acceleration is completed (6). Because there is considerable stress on the ligaments at the beginning of the lift, it is suggested that the erector spinae activity begin at the initial part of the lift to stabilize the back (48).
In the back lift, the person bends over at the waist with the knees straight, as in the “good morning” exercise. This exercise creates the highest shear and compressive stress on the lumbar vertebrae, but the erector spinae are much more active in this type of exercise (74). In the back lift, the movement is initiated by the hamstrings and the gluteus maximus and then followed up by activity from the erector spinae. Extension of the spine begins approximately one third of the way into the lift (6). In performing the back lift with no load, the erector spinae becomes active after the beginning of the lift, but if the lift is performed with weight, the erector spinae is active before the start of the lift (88).
In comparing leg lifts and back lifts, one must consider both the risks and the gains. The back lift imposes a greater risk of injury to the vertebrae because of the higher forces imposed on the system. Any stooping posture of the trunk imposes greater compression forces on the spine; consequently, a trunk flexion posture in a lift should be discouraged (23). The disc pressures are much higher in the back lift than in the leg lift, mainly because of the trunk position and distance (62). The erector spinae activity in the back lift is greater than that in the leg lift.
Trunk extensor activity increases with increases in trunk lean, and knee extension activity decreases as trunk lean decreases (55). Maximal erector spinae activity also occurs later in the back lift than the leg lift. Finally, abdominal activity is lower in the back lift than in the leg lift (88). Consequently, the back is not as well supported in the back lift as it is in the leg lift, creating additional potential for injury.
To work the extensors from the standing position by hyperextending the trunk requires an initial contribution from the erector spinae. This activity drops off and then picks up again later in the hyperextension movement (68). If resistance to the movement is offered, the activity in the lumbar erector spinae movement increases dramatically (43).
Trunk Rotators and Lateral Flexors
The rotation and lateral flexion movements of the trunk are not usually emphasized in an exercise program. Some examples of rotation and lateral bending exercises are provided inFigure 7-19. There is some benefit to including some of these exercises in a training routine because rotation is an important component of many movement patterns. Likewise, lateral flexion is an important component of activities such as throwing, diving, and gymnastics.
Some individuals try to isolate the obliques by performing trunk rotation exercises against external resistance. The obliques are not isolated in this type of exercise because the erector spinae muscles are also actively involved. If a rotation exercise is added to an exercise set, caution should be used. No combined exercises should be performed in which the trunk is flexed or extended and then rotated. This loads the vertebrae excessively and unnecessarily. If rotation is to be included, it should be done in isolation. The same holds true for lateral flexion exercises that can be performed against a resistance from a standing or sidelying position.
Many strength routines for the trunk incorporate the use of an exercise ball. The advantage of exercises using this kind of ball is the improvement in posture because of the ongoing spine stabilization that is required while sitting or balancing on the ball. Exercises can progress from easy to difficult depending on the distance of the ball from the body.
Flexibility and The Trunk Muscles
It is recommended that stretching exercises be functional range of motion activities that do not require extreme range of motions. In fact, with increased flexibility, there may be increased risk of injury (54). With this in mind, stretching the trunk muscles is easy and can be done from a standing or lying position. The lying positions offer stabilization of the lower extremity and the pelvis, which contribute to the movements if the exercise is performed from a stand. All stretching of the trunk muscles should be done through one plane only because movements through more than one plane at a time excessively load the vertebral segments.
Caution should be used in prescribing maximum trunk flexion exercises, such as touching the toes for the stretch of the extensors. Remember, the trunk is supported by the ligaments and the posterior elements of the segment in this position, and the loads on the discs are large, so an alternative exercise should be chosen.
Similarly, the sit-reach test is often used as a measure of both low-back and hamstring flexibility. It has been suggested that the sit-reach is primarily an assessment of hamstring, not low-back, flexibility (71). The sit-reach position has also been shown to increase the strain on the low back as a result of exaggerated posterior tilt of the pelvis. It is recommended that the sit-reach stretching exercise be done while maintaining a mild lordotic lumbar curve throughout to avoid the exaggerated curve.
Inflexibility in the trunk or posterior thigh influences the load and strains incurred during exercise. If the low back is inflexible, the reversal of the lumbar curve is restrained in forward flexion movements. This places an additional strain on the hamstrings. If the hamstrings are inflexible, rotation of the pelvis is restricted, placing additional strain on the low-back muscles and ligaments. Additionally, inhibition of forward rotation of the pelvis increases the overall compressive stress on the spine (71).
The core is the area between the sternum and the knees, and exercises to this area emphasize the abdominals, low back, and hips. The core muscles transmit forces between the upper and lower body and provide spinal stability during lifting and everyday activity (86). Strengthening the core muscles can serve as a preventive measure for back injury or the reoccurrence of a back injury. Core exercises that focus specifically on the lumbar region of the trunk are illustrated in Figure 7-21. The curl-up targets the rectus abdominus; the side bridge targets the obliques, transverse abdominus, and quadratus lumborum; and the bird dog targets the back and hip extensors (54). Lateral flexion exercises also stimulate coactivation of the extensors and the flexors (46). Hollowing and bracing the low back against the ground as well as the cat camel exercise are known to increase activity in the transverse abdominus and internal oblique (11). The front bridge or cat camel is done standing or on all fours.
Injury Potential of the Trunk
Back pain has been shown to affect as high as 17% of U.S. workers, with higher injury rates amongst occupations such as carpentry, construction, nurses, and dentists (36). and 85% of the population of the Western world reports back pain at some point in their lives, with peak incidence of injury in the working years (13). Low-back pain is a chronic problem for 1% to 5% of the general population and recurs in 30% to 70% of those with an initial low-back problem (71). The sexes are affected equally. Low-back pain is most common in the age range of 25 to 60 years, with the highest incidence of low-back pain at age 40 years (71). Back pain is uncommon in children and athletes. Back sprain accounts for only 2% to 3% of the total sprains
in the athletic population (25), but it is very debilitating. Back pain is a particular problem in sports that require high levels of bending and rotation, such as golf, gymnastics, and baseball. The major source of back pain is muscle or tendon strain, and only 1% to 5% of back pain is related to an injury of the intervertebral disc (14). Torn ligaments are not a common source of back pain, and most back injuries result from microtraumas to muscles and tendons from activities such as unbalanced lifting, prolonged static postures, chronic stress, or chronic sitting (14).
FIGURE 7-21 Exercises for the core focus on the abdominals and the trunk and hip extensors. Strengthening these muscles may prevent injury to the low back.
Back pain can be caused by compression on the spinal cord or nerve roots from an intervertebral disc protrusion or disc prolapse. Disc protrusions occur most frequently at the intervertebral junctions of C5-C6, C6-C7, L4-L5, and L5-S1 (45). Lumbar disc protrusions occur at a significantly higher rate than in any other region of the trunk. As shown in Figure 7-22, the disc protrusion may impinge on the nerve exiting from the cord, causing problems throughout the back and the lower extremity.
A disc injury commonly occurs to a motion segment that is compressed while being flexed slightly more than the normal limits of motion (3). Also, a significant amount of torsion, or rotation, of the trunk has been shown to tear fibers in the annulus fibrosus of the disc. Pure compression to the spine usually injures the vertebral bodies and end plates rather than the disc. Likewise, maximal flexion of the trunk without compression may injure the posterior ligaments of the arch rather than the disc (3).
In the case of a disc prolapse, the nucleus pulposus extrudes into the annulus fibrosus either laterally or vertically. Vertical prolapse is more common than posterior prolapse, and the result is an anterolateral bulge of the annulus. This causes the bodies to tilt forward and pivot on the apophyseal joints, placing stress on the facets (95). A posterior or posterolateral prolapse of the disc into the spinal canal creates back pain and neurologic symptoms via nerve impingement. Schmorl's nodes is a condition in which a vertical prolapse of part of the nucleus pulposus protrudes into an end plate lesion of the adjacent vertebrae (40). Damage to the disc is created through excessive load, failure of the inner posterior annulus fibers, or disc degeneration (10).
FIGURE 7-22 Injury to a disc can be caused by extreme trunk flexion while the trunk is compressed or loaded. Rotation movements can also tear the disc. When a disc is ruptured, pressure can be put on the spinal nerves.
Disc degeneration in the early elderly years consists of a gradual process during which splits and tears develop in the disc tissue. The progression of disc degeneration is illustrated inFigure 7-23. Although the symptoms of disc degeneration may not appear until the early elderly years, the process may begin much earlier in life. It is common for disc degeneration to begin as the posterior muscles and ligaments relax, compressing the anterior portion and putting tension on the posterior portion of the disc. The tears in the disc are usually parallel to the end plates halfway between the end plate and the middle of the disc (95). As these tears get larger, there is potential for separation of the central portion of the disc. The splits and tears usually occur in the posterior and posterolateral portions of the disc along the posterior border of the marginal edges of the vertebral bodies (95). Eventually, the tears may be filled with connective tissue and later with bone. Osteophytes develop on the periphery of the vertebral bodies, and cancellous bone is gradually laid down in the anterior portion of the disc where the pressure is great.
This condition can progress to a point of forming an osseous connection between two vertebral bodies that leads to further necrosis of the disc. Osteoarthritis of the apophyseal joints is also a byproduct of disc degeneration as added stress is placed on these joints. A disc that has
undergone slight degeneration is also more susceptible to prolapse (3).
FIGURE 7-23 Disc degeneration narrows the joint space, causing a shortening of the ligaments, increased pressure on the disc, and stress on the apophyseal joints.
FIGURE 7-24 A. A fatigue fracture to the pars interarticularis is called spondylolysis. B. When the fracture occurs bilaterally, spondylolisthesis develops.
Fractures of the various osseous components of the vertebrae can occur. The fractures can be in the spinous processes, transverse processes, or laminae, or they can be compressive fractures of the vertebral body itself Spondylolysis, shown in Figure 7-24, involves a fatigue fracture of the posterior neural arch at the pars interarticularis. This injury is most common in sports requiring repeated flexion, extension, and rotation, such as gymnastics, weightlifting, football, dance, and wrestling (76). There is a 20.7% incidence of spondylolysis in athletes (41).
A typical example of an athlete who may fracture the neural arch is the football lineman. The lineman assumes a starting position in a three- or four-point stance with the trunk flexed. This flattens the low back, compresses and narrows the anterior portion of the disc, and stresses the transverse arch. When the lineman drives up with trunk extension and makes contact with an opponent, a large shear force across the apophyseal joint is created (76).
Another example of a spondylolysis-causing activity is pole vaulting. The vaulter extends the trunk at the plant and follows with rapid flexion of the trunk (75). The large range of motion occurring with rapid acceleration and deceleration is responsible for the development of the stress fracture. This condition is usually associated with repetitive activities, seldom with a single traumatic event (10).
With spondylolysis on both sides, spondylolisthesis can develop (Fig. 7-24). With a bilateral defect of the neural arch, the motion segment is unstable, and the anterior and posterior elements separate. The top vertebrae slip anteriorly over the bottom vertebrae. This condition is most common in the lumbar vertebrae, especially at the site of L5-S1, where shear forces are often high. The condition worsens with flexion of the spine, which adds to the anterior shear on the motion segment (96).
The cervical, thoracic, and lumbar regions of the trunk are subject to their specific injuries. In the cervical region of the spine, flexion and extension injuries, or whiplash injuries, are common. In whiplash, the head is rapidly flexed, straining the posterior ligaments or even dislocating the posterior apophyseal joints if the force is large (82). The rapid acceleration and deceleration of the head causes both sprain and muscular strain in the cervical region. During a rear-end impact, the body is thrown forward and the head is forced into hyperextension. This is followed by a rapid jerk of the head forward into flexion. This forceful whiplash can fracture the vertebral bodies through the wedging action in the flexion movement, which compresses the bodies together. The seventh cervical vertebra is a likely site of fracture in a flexion injury.
Flexion and compression injuries are also common in the cervical vertebrae and are seen in sports such as football and diving. The cervical spine straightens with flexion, creating a columnlike structure that lacks flexibility when contact is made. The discs, vertebral bodies, process, and ligaments resist this load, and when capacity is exceeded, vertebral dislocation and spinal cord impingement can result.
Injuries to the cervical vertebrae as a result of forceful extension include rupture of the anterior longitudinal ligament and actual separation of the annulus fibers of the disc from the vertebrae. Forceful hyperextension of the spine can be a part of whiplash injury, usually affecting the sixth cervical vertebra (85).
The cervical vertebrae are susceptible to injury in certain activities that subject the region to repeated forces. In diving, high jumping, and other activities with unusual landing techniques, individuals are subjected to repeated extension and flexion forces on the cervical vertebrae that may cause an injury (10). Positioning and posture of the cervical vertebrae are important in many of these activities that involve external contact in the region.
The thoracic region of the spine is not injured as frequently as the cervical and lumbar regions, probably because of its stabilization and limited motion as a result of interface with the ribs. The condition called Scheuermann's disease is commonly found in the thoracic region. This disease is an increase in the kyphosis of the thoracic region from wedging of the vertebrae. The cause of Scheuermann's disease is unknown, but it appears to be most prevalent in individuals who handle heavy objects. It is also common among competitive butterfly-stroke swimmers (10).
The lumbar region of the spine is the most injured, primarily because of the magnitude of the loads it carries. Low-back pain can originate in any of a number of sites in the lumbar area. It is believed that in a sudden onset of pain, muscles are usually the problem, irritated through some rapid twisting or reaching movement. If the pain is of the low-grade chronic type, overuse is seen as the culprit (96).
Myofascial pain, common in the low back, involves muscle sheaths and tendons that have been strained as a result of some mechanical trauma or reflex spasm in the muscle (98). Muscle strain in the lumbar region is also related to the high tensions created while lifting from a stooped position.
Muscle spasms over time produce a dull, aching pain in the lumbar region. Likewise, a dull pain can be caused by distorted postures maintained for long periods. The
muscles fatigue, the ligaments are stressed, and connective tissue can become inflamed as a result of poor posture.
Irritation of the joints in the lumbar region occurs most often in activities that involve frequent stooping, such as gardening and construction. Abnormal stress on the apophyseal joints is also common in activities such as gymnastics, ballet, and figure skating (98). Both spondylolysis and spondylolisthesis occur more frequently in the lumbar region than any other region of the trunk.
The intervertebral discs in the lumbar region have a greater incidence of disc prolapse than any other segment of the spinal column. A disc protrusion, as in any other area of the trunk, may impinge on nerve roots exiting the spinal cord, creating numbness, tingling, or pain in the adjacent body segments. Sciatica is such a condition. In it, the sciatic nerve is compressed, sending pain down the lateral aspect of the lower extremity.
The cause of low-back pain is not clearly defined because of the multiple risk factors associated with the disorder. Some of these factors are repetitive work; bending and twisting; pushing and pulling; tripping, slipping, and falling; and sitting or static work posture (71). A low-back injury can be created through some uncoordinated or abnormal lift or through repetitive loading over time.
Low-back pain associated with standing postures is related to positions maintaining hyperextension of the knees, hyperlordosis of the lumbar vertebrae, rounded shoulders, or hyperlordosis of the cervical vertebrae. In the seated posture, it is best to avoid crossing the legs at the knees because this position places stress on the low back. Likewise, positions that maintain the legs in an extended position with the hips flexed should be avoided because they accentuate lordosis in the low back.
FIGURE 7-25 Low-back injury can be reduced if proper lifting techniques are used. The most important consideration is not whether the person uses the legs but where the weight is with respect to the body. Proper lifting technique has the weight close to the body with the head up and the back arched (A). The leg lift technique (B) is no better than the back lift (C) if the weight is held far from the body. Both B and C should be avoided.
Low-back injuries as a result of lifting are primarily a consequence of the weight of the load and its distance from the body. A correct lifting posture, as mentioned earlier in this chapter, is one with the back erect, knees bent, weight close to the body, and movement through one plane only (Fig. 7-25). This lifting technique minimizes the load imposed on the low back. A stooped lifting posture reduces the activity of the trunk extensors, and the forward moment is resisted by passive structures such as the discs, ligaments, and fascia. Lifting with flexed posture can place as much as 16% to 31% of the extensor moment on the passive structures (27), placing them at risk for injury.
A sudden maximal effort in response to an unexpected load is related to high incidence of back injury (49). The back extensors are slow postural muscles that may not generate force rapidly enough to prevent excessive spine bending or twisting with the application of a sudden load. Unexpected loading can also increase the compressive force when the extensors contract to prevent a postural disturbance when a weight is unexpectedly placed in the hands (49). This could lead to a combination of high compression and bending stresses on the vertebrae.
Muscle strength and flexibility are also seen as predisposing factors for low-back pain. Tight hamstrings and an inflexible iliotibial band have both been associated with low-back pain (71). Weak abdominals are also related to low-back pain. If the abdominals are weak, control over the pelvis is lacking, and hyperlordosis will prevail. The hyper-lordotic position puts undue stress on the posterior apophyseal joints and the intervertebral disc. This is an important consideration in an activity such as a sit-up or curl-up.
Erector spinae muscle activity has also been shown to relate to incidence of low-back pain. Individuals with low-back pain also have increased electrical activity and fatigue
in the erector spinae muscle group (71). Even though there are inverse relationships between strength and flexibility and low-back pain, the strength and flexibility of an individual may not predict whether that person will have low-back pain. However, strength, flexibility, and fitness are predictive of the recurrence of low-back pain (71).
Effects of Aging on the Trunk
The effects of aging on the spine may predispose someone to an injury or painful condition. During the process of aging, the flexibility of the spine decreases to as little as a tenth of that of younger individuals (61). There is also a corresponding loss of strength in the trunk muscles of approximately 1% per year (71). Between ages 30 and 80 years, the strength in cartilage, bone, and ligaments reduces by approximately 30%, 20%, and 18%, respectively (71).
The shape and length of the spine also change with aging. There is a smaller fluid region in the aging disc that places more stress on the annulus fibrosus (26). The discs may also lose height and create a shorter spine, although it has been reported that the ventral disc height is constant in both men and women in the age range of 16 to 57 years (32). There is also an increase in lateral bending of the trunk, an increase in thoracic kyphosis, and a decrease in lumbar lordosis (58). In the lumbar region specifically, there is a loss of mobility in the L5-S1 segment with an accompanying increase in the mobility of the other segments (40). It is not clear whether these age-related changes are a normal process of aging or are associated with abuse of the trunk, disuse of the trunk, or are disease related. It is clear that there is benefit in maintaining strength and flexibility in the trunk well into the elderly years.
Contribution of the Trunk E Musculature to Sports Skills or Movements
The contribution of the back muscles to lifting has been presented in an earlier discussion. Likewise, the contribution of the abdominals to a sit-up or curl-up exercise was evaluated. The trunk muscles also contribute to activities such as walking and running.
At touchdown, the trunk flexes toward the side of the limb making contact with the ground. It also moves back, and both of these movements are maximum at the end of the double support phase. After moving into single support, the trunk moves forward while still maintaining lateral flexion toward the support limb (91). As the speed of walking increases, there is a corresponding increase in lumbar range of motion accompanied by higher muscle activation levels (16).
For running, the movements in the support phase are much the same, with trunk flexion and lateral flexion to the support side. One difference is that whereas in walking, there is trunk extension at touchdown, in running, the trunk is flexed at touchdown only at fast speeds (90). At slower speeds, the trunk is extended at touch-down. For a full cycle in both running and walking, the trunk moves forward and backward twice per cycle.
Another difference between walking and running is the amount and duration of lateral flexion in the support phase. In running, the amount of lateral flexion is greater, but lateral flexion is held longer in the maximal position in walking than in running (91). There is one full oscillation of lateral flexion from one side to the other for every walking and running cycle.
As contact is made with the ground in both running and walking, there is a burst of activity in the longissimus and multifidus muscles. This activity can begin just before contact, usually as an ipsilateral contraction to control the lateral bending of the trunk. It is followed by a contraction of the contralateral erector spinae muscles, so that both sides contract (90).
There is a second burst of activity in these muscles in the middle of the cycle, occurring with contact of the other limb. Here, both the longissimus and the multifidus are again active. In the first burst of activity, the ipsilateral muscles are more active, but in this second burst, the contralateral muscles are more active (90). The activity of the erector spinae muscles coincides with extensor activity at the hip, knee, and ankle joints.
The lumbar muscles serve to restrict locomotion by controlling the lateral flexion and the forward flexion of the trunk (90). Cervical muscles serve to maintain the head in an erect position on the trunk and are not as active as the muscles in other regions of the spine.
A more thorough review of muscular activity is provided for a topspin tennis serve (Fig. 7-26). There is considerable activity in the abdominals and the erector spinae in the tennis serve. The most muscular activity is in the descending wind up and the acceleration phase (21). There is also considerable coactivation of the erector spinae and the abdominals to stabilize the trunk when it is brought back in a back arch in the descending windup and the subsequent acceleration. Both the internal and external oblique muscles are the most active of the trunk muscles. Because both the erector spinae and the abdominals are responsible for lateral flexion and rotation, there is unilateral activation of muscles to initiate left trunk lateral flexion and rotation to the right and left.
Forces Acting at Joints in the Trunk
Loads applied to the vertebral column are produced by body weight, muscular force acting on each motion segment, prestress forces caused by disc and ligament forces,
and external loads being handled or applied (48). The spine cannot support more than 20 N without muscular contraction (4,64). The muscleless lumbar spine can withstand a somewhat higher force ([gt]100 N) before buckling (64).
The discs, apophyseal joints, and intervertebral ligaments are the load-bearing structures. Compressive forces are applied perpendicular to the disc; thus, the line of action varies with the orientation of the disc. For example, in the lumbar vertebrae, only at the L3-L4 level is the compressive force vertical in upright standing (26). Compression forces are primarily resisted by the disc unless there is disc narrowing, where the resistance is offered by the apophyseal joints (26). For flexion bending moments, 70% of the moment is resisted by the intervertebral ligaments and 30% by the discs and in extension, and two thirds of the moment is resisted by the apophyseal joints and the neural arch and a third by the discs (26). Lateral bending moments are resisted by the discs, and rotation is resisted by the discs and bony contact at the apophyseal joints (26).
The lumbar vertebrae handle the largest load, primarily because of their positioning, the position of the center of mass relative to the lumbar region, and greater body weight acting at the lumbar region than other regions of the spine. Of the compressional load carried by the lumbar vertebrae, 18% is a result of the weight of the head and trunk (57). The other source of substantial compression is muscle activity. Muscle forces protect the spine from excessive bending and torsion but subject the spine to high compressive forces. The compressive forces are increased with more lumbar flexion, and it is fairly common to see substantial increases in lumbar flexion with actions such as crossing legs (35% to 53%), squatting on the heels (70% to 75%), lifting weights from the ground (70% to 100%), and rapid lunging movements (100% to 110%) (26).
FIGURE 7-26 Trunk muscles involved in the top-spin tennis serve showing the level of muscle activity (low, moderate, high) and the type of muscle action (concentric [CON] and eccentric [ECC]) with the associated purpose.
The axial load on the lumbar vertebrae in standing is 700 N. This can quickly increase to values greater than 3000 N when a heavy load is lifted from the ground and can be reduced by almost half in the supine position (300 N) (15). Fortunately, the lumbar spine can resist approximately 9800 N of vertical load before fracturing (61).
The load on the lumbar vertebrae is more affected by distance of the load from the body than the actual posture of lifting (61). For example, the magnitude of the compressive force acting on the lumbar vertebrae in a half squat is 6 to 10 times body weight (18). If the weight is taken farther as a result of flexion, compressive loads increase, even with postural adjustments such as flattening the lumbar curve (29).
Loads acting at the lumbar vertebrae can be as high as 2 to 2.5 times body weight in an activity such as walking (17). These loads are maximum at toe-off and increase with an increase in walking speed. Loads on the vertebrae in an activity such as walking are a result of muscle activity in the extensors and the amount of trunk lean in the walker (48). This is compared with loads of more than 4 body weights in rowing, which is maximum in the drive phase as a result of muscle contraction and trunk position (59).
The direction of the force or load acting on the vertebrae is influenced by positioning. In a standing posture with the sacrum inclined 30° to the vertical, there is a shear force acting across the lumbosacral joint that is approximately 50% of the body weight above the joint (Fig. 7-27). If the sacral angle increases to 40°, the shear force increases to 65% of the body weight, and with a 50° sacral inclination, the force acting across the joint is 75% of the body weight above the joint (77).
Lumbosacral loads are also high in exercises such as the squat, in which maximum forces are generated at the so-called “sticking point” of the ascent. These loads are higher than loads recorded at either the knee or the hip for the same activity (65).
Loads are applied to the lumbar vertebrae even in a relaxed supine position of repose. The loads are significantly reduced because of the loss of the body weight forces but still present as a result of muscular and ligamentous forces. In fact, the straight-leg lying position imposes load on the lumbar vertebrae because of the pull of the psoas muscle. Flexing the thigh by placing a pillow under the knees can reduce this load.
FIGURE 7-27 A. The shear force across the lumbosacral joint in standing is approximately 50% of the body weight. B. If one flexes to where the sacral angle increases to 50°, the shear force can increase to as much as 75% of the body weight above the joint.
Loads imposed on the vertebrae are carried by the various structural elements of the segment. The articulating facets carry large loads in the lumbar vertebrae during extension, torsion, and lateral bending but no loads in flexion (79). Facet loads in extension have been shown to be as high as 30% to 50% of the total spine load, and in arthritic joints, the percentage can be higher (38). The posterior and anterior ligaments carry loads in flexion and extension, respectively, but carry little load in lateral bending and torsion. The intervertebral discs absorb and distribute a great proportion of the load imposed on the vertebrae. The intradiscal pressure is 1.3 to 1.5 times the compressional load applied per unit area of disc (61,80), and the pressure increases linearly with loads up to 2000 N (60). The load on the third lumbar vertebra in standing is approximately 60% of total body weight (62).
Pressures in sitting are 40% more than those in standing, but standing interdiscal pressures can be reduced by placing one foot in front of the other and elevating it (85). Whereas in standing, the natural curvature of the lumbar spine is increased, the curvature is reduced in sitting. The increased curvature reduces the pressure in the nucleus pulposus while at the same time increasing loading of the apophyseal joints and increasing compression on the posterior annulus fibrosus fibers (1). Pressures within the disc are large with flexion and lateral flexion movements of the trunk and small with extension and rotation (63). The pressure increases can be attributed to tension generated in the ligaments, which can increase intradiscal pressure by 100% or more in full flexion (1). The lateral bend produces larger pressures than flexion and even more pressure if rotation added to the side bend causes asymmetrical bending and compression (8).
Intervertebral discs have been shown to withstand compressive loads in the range of 2500 to 7650 N (69). In older individuals, the range is much smaller, and in individuals younger than 40 years, the range is much larger (69).
The posterior elements of the spinal segment assist with load bearing. When the spine is under compression, the load is supported partially by the pedicles and pars interarticularis and somewhat by the apophyseal joints. When compression and bending loads are applied to the spine, 25% of the load is carried by the apophyseal joints. Only 16% of the loads imposed by compressive and shear forces are carried by the apophyseal joints (57). Any extension of the spine is accompanied by an increase in the compressive strain on the pedicles, an increase in both compressive and tensile strain in the pars articularis, and an increase in the compressive force acting at the apophyseal joints (42).
In full trunk flexion, the loads are maintained and absorbed by the apophyseal capsular ligaments, intervertebral disc, supraspinous and interspinous ligaments, and ligamentum flavum, in that order (3). The erector spinae muscles also offer some resistance passively.
In compression, most of the load is carried by the disc and the vertebral body The vertebral body is susceptible to injury before the disc and will fail at compressive loads of only 3700 N in the elderly and 13,000 in a young, healthy adult (47). In rotation, during which torsional forces are applied, the apophyseal joints are more susceptible to injury. During a forward bend movement, the disc and the apophyseal joints are at risk for injury because of compressive forces on the anterior motion segment and tensile forces on the posterior elements.
Loads in the cervical region of the spine are lower than in the thoracic or lumbar region and vary with position of the head, becoming significant in extreme positions of flexion and extension (82). Loads on the lumbar disc have been calculated using a miniaturized pressure transducer (61). Approximate loads for various postures and exercises are presented inFigure 7-28 although the researchers
recommend caution about the interpretation of absolute values and direct more attention to the relative values (61). Studies have demonstrated that the compressive load on the low back can be greater than 3000 N in exercises such as sit-ups, and a sit-up with the feet fixed results in similar loads whether the bent-knee or straight-leg technique is used (52).
FIGURE 7-28 The representative postures or movements are shown in order of calculated load on the lumbar vertebrae using a miniaturized pressure transducer. The standing posture imposed the least amount of load (686 N) (A), followed by the double straight-leg raise (1176 N) (B), back hyperextension (1470 N) (C), sit-ups with knees straight (1715 N) (D), sit-ups with knees bent (1764 N) (E), and bending forward with weight in the hands (1813 N) (F). (Adapted with permission from
Nachemson, A. . Lumbar intradiscal pressure. In M. Jayson [Ed.]. The Lumbar Spine and Back Pain. Kent: Pitman Medical.
The vertebral column provides both flexibility and stability to the body. The four curves–cervical, thoracic, lumbar, and sacral–form a modified elastic rod. The cervical, thoracic, and lumbar curves are mobile, and the sacral curve is rigid.
Spinal column movement as a whole is created by small movements at each motion segment. Each motion segment consists of two adjacent vertebrae and the disc separating them. The anterior portion of the motion segment includes the vertebral body, intervertebral disc, and ligaments. Movement is allowed as the disc compresses. Within the disc itself, the gel-like mass in the center, the nucleus pulposus, absorbs the compression and creates tension force in the annulus fibrosus, the concentric layers of fibrous tissue surrounding the pulposus.
The posterior portion of the motion segment includes the neural arches, intervertebral joints, transverse and spinous processes, and ligaments. This portion of the motion segment must accommodate large tensile forces.
The range of motion in each motion segment is only a few degrees, but in combination, the trunk is capable of moving through considerable range of motion. Flexion occurs freely in the lumbar region through 50° to 60° and the total range of flexion motion is 110° to 140°. Lateral flexion range of motion is approximately 75° to 85°, mainly in the cervical and lumbar regions, with some contribution from the thoracic region. Rotation occurs through 90° and is free in the cervical region. Rotation takes place in combination with lateral flexion in the thoracic and lumbar region.
Most lumbar spine movements are accompanied by pelvic movements, termed the lumbopelvic rhythm. In trunk flexion, the pelvis tilts anteriorly and moves backward. In trunk extension, the pelvis moves posteriorly and shifts forward. The pelvis moves with the trunk in rotation and lateral flexion.
The extension movement of the trunk is produced by the erector spinae and the deep posterior muscles running in pairs along the spinal column. The extensors also are very active, controlling flexion of the trunk through the first 50° to 60° of a lowering action with gravity. The abdominals produce flexion of the trunk against gravity or resistance. They also produce rotation and lateral flexion of the trunk with assistance from the extensors.
The trunk muscles can generate the greatest amount of strength in the extension movement, but the total extensor moment is only slightly greater than flexor moment. The strength output is also influenced by trunk position. In lifting, the extensor contribution diminishes the farther the object is horizontally from the body. The contribution of the various segments and muscles is also influenced by the angle of pull and the width of the object being lifted.
Posture and spinal stabilization is an important consideration in the maintenance of a healthy back. The spine is stabilized by three systems: a passive system, an active musculoskeletal system, and a neural feedback system. The transverse abdominus, erector spinae, and internal oblique play important roles in spinal stabilization. Both standing and sitting postures require some support from the trunk muscles. In the workplace, posture becomes an important factor, particularly if static positions are maintained for long periods of time. It is suggested that short breaks occur regularly over the course of the work day to minimize the accumulative strain in static postures. Postures that should be avoided include a slouched standing posture, prolonged sitting, unsupported sitting, and continuous flexion positions.
Postural deviations are common in the general population. Some of the common postural deviations in the trunk are excessive lordosis, excessive kyphosis, and scoliosis. The most serious of these is scoliosis.
Conditioning of the trunk muscles should always include exercises for the low back. Additionally, trunk exercises should be evaluated in terms of safety and effectiveness. For example, the trunk flexors can be strengthened using a variety of trunk or hip flexion exercises including the sit-up, curl-up, or the double leg raise. Conditioning of the extensors can be done through the use of various lifts. Both the leg lift and the back lift are commonly used to strengthen the extensors. The back lift imposes more stress on the vertebrae and produces more disc pressure than the leg lift.
Stretching of the trunk muscles can be done either standing or lying, but it is recommended that stretching occur through a functional range. The lying position offers more support for the trunk. Toe-touch exercises for flexibility should be avoided because of the strain to the posterior elements of the vertebral column.
The incidence of injury to the trunk is high, and it is predicted that 85% of the population will have back pain at some time in their lives. Back pain can be caused by disc protrusion or prolapse on a nerve but is more likely to be associated with soft tissue sprain or strain. Disc degeneration occurs with aging and may eventually lead to reduction of the joint space and nerve compression. The spinal column can also undergo fractures in the vertebral body as a result of compressive loading or in the posterior neural arch associated with hyperlordosis (spondylolysis). When the defect occurs on both sides of the neural arch, spondylolisthesis develops: The vertebrae slip anteriorly over each other. Some injuries are specific to regions of the trunk, such as whiplash in the cervical region and Scheuermann's disease in the thoracic vertebrae.
Changes in the spine associated with aging include decreased flexibility, loss of strength, loss of height in the spine, and an increase in lateral bending and thoracic kyphosis. It is not clear whether these changes are a normal consequence of aging or are related to disuse, misuse, or a specific disease process.
The contribution of the muscles of the trunk to sport skills and movements is important for balance and stability. The trunk muscles are active in both walking and running as the trunk laterally flexes, flexes and extends, and rotates. There is also considerable activity in the cervical region of the trunk as the head and upper body are maintained in an upright position. In a tennis serve, unilateral contraction of the abdominal and erector spinae occurs to initiate lateral flexion and rotation actions in the tennis serve. The obliques are the most active trunk muscle in the tennis serve.
The loads on the vertebrae are substantial in lifting and in different postures. The loads on the lumbar vertebrae can range from 2 to 10 times body weight in activities such as walking and weight lifting. Loads on the actual intervertebral discs are influenced by a change in posture. For example, the pressures on the disc are 40% more in sitting than standing.
True or False
A combination of muscles, including rectus abdominis, internal oblique, external oblique, and transverse abdominis; flexors and rotators of the trunk.
Ring of fibrocartilage that runs in concentric layers around the nucleus pulposus in the intervertebral disc; absorbs tensile stress as the disc is compressed.
Anterior Longitudinal Ligament
Ligament inserting from the sacrum, anterior vertebral body and disc up to the atlas; limits hyperextension and forward sliding of the vertebrae.
Synovial joints between adjacent vertebrae, connected at the superior and inferior facets on the laminae.
Articulation between the atlas and the axis.
The articulation between the atlas with the occipital bone of the skull.
The first cervical vertebra; articulates with the occipital bone.
The second cervical vertebra.
The neck region of the trunk, consisting of seven vertebrae.
The vertebral region where the cervical curve ends and the thoracic curve begins; C7-T1.
Ligament inserting on the tubercles of the ribs, transverse process of the vertebrae; supports rib attachment to thoracic vertebrae.
Ligament inserting on the odontoid bone and arch of the atlas; stabilizes the odontoid and atlas; prevents posterior movement of dens in atlas.
Toothlike process projecting from the superior surface of the axis; articulating surface with the atlas; also called odontoid process.
Gradual breakdown of the intervertebral disc in which splits and tears develop.
Injury to the intervertebral disc in which the nucleus pulposus extrudes into the annulus fibrosus.
A combination of muscles, including the iliocostalis, longissimus, and spinalis muscles; extensors of the trunk.
Muscle inserting on ribs 9-12, anterior, superior spine, pubic tubercle, anterior iliac crest; flexes, laterally flexes, and rotates the trunk to the opposite side.
Muscle inserting on ribs 3-6, transverse process of C4-C6; extends, laterally flexes, and rotates the cervical region of the trunk to the same side.
Muscle inserting on the sacrum, spinous processes of L1-L5, T11, T12, iliac crest, lower six ribs; extends, laterally flexes, and rotates the thoracic region of the trunk to the same side.
Muscle inserting on the lower six ribs, upper six ribs, transverse process of C7; extends, laterally flexes, and rotates the thoracic region of the trunk of the same side.
Ligament inserting on the transverse process of L5 to the iliac crest; limits lumbar flexion and rotation.
Two muscles, the iliacus and the psoas, that insert on the bodies of T12 and L1-L5; the transverse processes of L1-L5; and the inner surface of ilium, sacrum, and lesser trochanter; flexes the trunk and thigh.
Muscle inserting on the iliac crest, lumbar fascia, ribs 8-10, and linea alba; flexes, laterally flexes, and rotates the trunk to the same side.
Muscle inserting on the spinous processes; extends and hyperextends the trunk.
Ligament inserting on the spinous processes; limits flexion of the trunk; limits shear forces on the vertebrae.
Muscles inserting on the transverse processes; extend and laterally flex the trunk.
Ligament inserting on the transverse processes; limits lateral flexion of the trunk.
Layers of fibrocartilage between the adjacent bodies of the vertebrae; a fibrous ring with a pulposus center.
A passage through the vertebrae formed by the inferior and superior notches on the pedicles; pathway for spinal nerves.
Increase in the convexity of the vertebral curve to the posterior.
One of the paired dorsal parts of the vertebral arch, connecting to the pedicles.
Ligament inserting on the lamina; limits flexion of the trunk, creates extension of the trunk, and creates tension in the disc.
Ligament inserting on the lamina; connects with the supraspinous ligament; limits cervical flexion, assists in cervical extension, creates tension in the disc.
Muscle inserting on the transverse processes of T1-T5, C4-C7, and mastoid process; extends, laterally flexes, and rotates the trunk.
Muscle inserting on the transverse processes of T1-T5 and C4-C6; extends, laterally flexes, and rotates the trunk to the same side.
Muscle inserting on the transverse processes of L1-L5, thoracolumbar fascia, transverse process of T1-T12; extends, laterally flexes, and rotates the trunk to the same side.
Muscle inserting on the transverse processes of C3-C6 and the occipital bone; flexes the head and the cervical region of the trunk, laterally flexes the trunk.
Longus Cervicis, Longus Colli
Muscle inserting on the trans-verse processes of C3-C5, bodies of T1-T2, bodies of C5-C7, T1-T3, atlas, transverse processes of C5-C6, and bodies of C2-C4; flexes and laterally flexes the cervical region of the trunk.
Increase in the anterior concavity of the vertebral curve.
Increase in the lumbar curve; swayback.
The region of the trunk between the thorax and the pelvis, consisting of five vertebrae.
The movement relationship and synchronization between the pelvis and the lumbar vertebrae.
The site on the vertebrae where the lumbar curve ends and the sacral curve begins; L5 and S1.
Muscle inserting on the sacrum, iliac spine, transverse processes of L5-C4, and spinous processes; extends, laterally flexes, and rotates the trunk to the opposite side.
Protective arch for the spinal cord, formed by the laminae and pedicles; also called the vertebral arch.
Spherical gel-like mass in the middle of the intervertebral disc; resists compressive forces applied to the spine.
Toothlike process projecting from the superior surface of the axis; articulating surface with the atlas; also called dens.
A site on the posterior neural arch.
A paired stem that connects the lamina to the vertebral body; part of the vertebral or neural arch.
Posterior Longitudinal Ligament
Ligament inserting on the posterior vertebral bodies and discs of the vertebrae; limits flexion of the trunk.
Muscle inserting on the iliac crest, transverse process of L1-L5, and the last rib; laterally flexes the trunk.
Ligament inserting on the head of the ribs and body of the vertebrae; holds the ribs to the vertebrae.
Muscle inserting on the transverse processes and laminae; extends and rotates the trunk to the opposite side.
Muscle inserting on the transverse process of cervical vertebrae and ribs 1 and 2; flexes and laterally flexes the cervical region of the trunk.
Necrosis and recalcification of the vertebrae; increase in kyphosis of the thoracic region because of vertebral wedging.
Vertical prolapse of part of the nucleus pulposus into an end plate lesion of an adjacent vertebra.
A lateral curve of the spine.
Muscle inserting on the facets of C4-C6, transverse processes of C7 to base of occipital; extends and laterally flexes the trunk.
Muscle inserting on the transverse processes of T1-T6 and spinous processes of C1-C5; extends, laterally flexes, and rotates the trunk.
Muscle inserting on the transverse processes of T6-T10 and spinous processes of T1-T4, C6, and C7; extends, laterally flexes, and rotates the trunk.
Muscle inserting on the spinous process of C7 to the Schmorl's nodes of the axis; extends and laterally flexes the trunk.
Muscle inserting on the spinous processes of L1, L2, T11, T12, and spinous processes of T1-T8; extends and laterally flexes the trunk.
A posterior projection from each vertebra, exiting at the arch.
Muscle inserting on the ligamentum nuchae; spinous processes of C7, T1-T3, mastoid process; and occipital bone; extends, laterally flexes, and rotates the cervical region of the trunk to the same side.
Muscle inserting on the spinous processes of T3-T6 and the transverse processes of C1-C3; extends, laterally flexes, and rotates the cervical region of the trunk to the same side.
Forward displacement of one vertebra over another; bilateral defect at the pars interarticularis site.
Fatigue fracture of the posterior neural arch of the vertebrae at the pars interarticularis site.
Muscle inserting on the sternum, clavicle, and mastoid process; flexes the head and cervical vertebrae and laterally flexes and rotates the cervical region of the trunk to the same side.
Ligament inserting on the spinous processes; limits trunk flexion, resists forward shear forces on the spine.
Increase in the thoracic curve; “hunchback.”
The region of the vertebrae where the thoracic curve ends and the lumbar curve begins; T12 and L1.
The chest or rib area, consisting of 12 vertebrae.
Projection on both sides of each vertebra; projects from the junction of the laminae and the pedicles.
Muscle inserting on the last six ribs, iliac crest, inguinal ligament, lumbodorsal fascia, linea alba, and pubic crest; increases intraabdominal pressure.